The Environmental Protection Agency (EPA) has recently realized that nearly 80% of the VOCs they are attempting to measure during site investigations are escaping from the sample prior to analysis. The possible sources of VOC escape are (1) during the sampling process, and (2) during the sampling holding time prior to analysis.
Sampling is currently being accomplished by attaching a vacuum pump to plastic tubing running to a filtering point at a given depth. Soil gas samples are drawn through the tubing and into gas sample bags. Locations of VOC loss are (1) through the plastic tubing, (2) through the sample bag walls, and (3) through any fittings. To improve the system and reduce the possible locations of leaks, both the sampling transport mechanisms to the laboratory and the sampling procedure need to be improved.
Currently, measurements of some sample properties are made prior to bringing the sample to the surface. Samples potentially change characteristics during the process of bringing them to the surface and the subsurface measurements purportedly allow corrections to be made to the surface measurements by comparisons to the subsurface measurements. However, all measurements necessary to determine the total nature of the sample cannot be made downhole so the accuracy of the corrections is conjectural. In addition, it is now common to have loss of volatiles from the sample in all current sampling devices.